The present invention relates to an optical network and, more particularly, a hardware implementation of the APON protocols, a direct digital drive of a laser diode, and measurement and control system for the direct digital drive of the laser diode.
A passive optical network (PON) is an optical network that distributes signals to multiple terminal devices using passive splitters without active electronics, such as, for example, repeaters. Conventionally, signal delivery over passive networks uses a variety of transfer protocols, such as, for example, a synchronous optical network (SONET) or an asynchronous transfer mode (ATM) protocols. From time to time, the International Telecommunication Union (ITU) issues recommendations and standards for PONs under standard G.983.1, which standard is incorporated herein by reference. Generally, the present invention is described with relation to APON, asynchronous transfer mode passive optical networks and the associated protocols, but one of ordinary skill in the art would understand that the use of APON is illustrative of the present invention and the present invention could be used for other types of passive optical networks, for example, EPON.
FIG. 1 illustrates a conventional APON 100. APON 100 could be either a fiber to the building (FTTB) or a fiber to the home (FTTH) network configuration. Generally, the PON system comprises an optical line terminal (OLT) 102, at least one optical network unit (ONU) 104, and at least one network termination (NT) 106 where an end user can access the system using, for example, a conventional computer, processor, or the like (not specifically shown). Connections 108o from the OLT 102 to the ONU 104 are fiber or optical connections and connections 108e from the ONU 104 to the NTs 106 are electrical connections. Depending on the number of ONUs 104 and NTs 106, one or more optical distribution nodes (ODN, a.k.a optical splitters and combiners) 110 may be situated between OLT 102 and ONU 104. Generally, ONUs 104 and NTs 106 reside at the end user or subscriber location (not specifically shown).
FIG. 9 illustrates a laser diode 902 using a conventional power source control system 904. As shown, laser diode 902 emits useful light 906 and rear facet light 908. Useful light 906 refers to light transmitted to connection 108O. A light monitor 910, which could be a photodiode, a light meter, or the like, senses the intensity of rear facet light 908. The intensity of rear facet light 908 corresponds to the intensity of useful light 906. Substantially simultaneously with sensing the intensity of rear facet light, light monitor 910 supplies a light level feedback signal to a laser power controller 912. Laser power controller 912 supplies a zero level current data signal 914 to a first programmable current source 916. First programmable current source 916 supplies the current necessary to drive laser diode 902 at the light intensity that corresponds to a logic level zero. Laser power controller 912 also supplies a modulation current data signal 918 to a second programmable current source 920. The modulation current data signal 918 determines the light intensity of the useful light output 906. A modulation signal 922 is supplied to the gate of a bi-stable switch 924 to turn the switch on and off based on whether the useful light intensity 906 should be at the logic 1 or the logic 0 intensity. The bi-stable switch passes current from the programmable current source 920 to be summed with the current from programmable source 916. The sum of the two currents drives laser diode 902. The feedback signal to laser power controller 912 allows fine-tuning of the drive currents so the average intensity of the light signal remains within the protocol requirements for logic levels 1 and 0. These current vary widely from laser diode to laser diode ranging from as low as 2 or 3 milliamps to as high as 50 to 60 milliamps.
FIG. 10 is a diagrammatic representation of useful light intensity to drive current. In particular, FIG. 10 shows transmission of an information cell 1002. As is known in the art, cell 1002 is an ATM protocol for transmitting information. FIG. 10 (and FIG. 11) does not actually represent transmission of a complete cell of information, but rather a short burst of information for convenience. Cell 1002a represents drive current for exemplary cell 1002 and cell 1002b represents light intensity for exemplary cell 1002. As shown, cell 1002a can be considered in discrete parts 1004a and 1004b. Part 1004a is the drive current necessary for the transmission of light bearing information having an intensity of logic 1s and 0s. Part 1004b is the drive current for the transmission of light having an intensity of logic 0 to allow for a zero level measurement only; in other words, no information is being transmitted during the zero level measurement. The duration and timing of part 1004b is generally controlled by the associated transmission protocols. Similarly, light intensity shown by cell 1002b over the course of cell 1002 transitions between the high and low drive currents for the laser diode. As the diagram shows, because of difficulties in controlling the drive current for the laser diode, first logic pulse 1006 is typically wasted adjusting the drive current for the existing operating conditions and temperatures. Part of the difficulty of controlling current occurs because the lasing cavity needs to charge the photons sufficiently to begin emitting light. Also, when the photons in the lasing cavity are sufficiently charged to the threshold or knee level, the laser emits a burst of light and oscillates until the photons are properly charged and the laser is correctly operating above the threshold level.
As can be seen from FIG. 10, during non-transmission period 1008, laser diode 902 is driven at a 0 current. Laser diode 902 is driven with a 0 current to inhibit the accidental transmission of light from laser diode 902 when laser diode 902 does not have a transmission grant. The drive current for a logic level 0 light intensity is some current greater than 0 current. Thus, one reason the first logic pulse 1006 is wasted is that time during the transmission of cell 1002 is required to charge the photons in the laser. While maintaining the laser drive current at the zero logic drive current (which is greater than 0 amps) would maintain the laser charged, it might allow for inadvertent light emission from the laser, which would cause interference with other transmitting lasers.
Transmission of a cell of information will be further explained with reference to FIGS. 1-3. Using ATM protocols, OLT 102 receives incoming cells 202 of information from a transport network or service node 112 destined for NTs 106. For simplicity, this example has three cells of information ABC destined for three separate NTs 106. The transport network could be any style network, such as the Internet, a Plain Old Telephone Service (POTS), digital video and/or audio streams. OLT 102 routes the incoming cell 202 over optical connection 108o through ODN 110 to three ONUs 1041-3. Using conventional protocols associated with APON, ONU 1041-3 selects the data for its associated NT 106 and converts the optical signal to an electrical signal for distribution to the NT 106 over connection 108e. For example, ONUa selects data cell A from incoming cell 202 and converts that data into an electrical signal for NT 106.
FIG. 3 shows the transmission of outgoing information from two NTs 1064 and 1065, for example. NT 1064 transmits an outgoing data cell D and NT 1065 transmits an outgoing data cell E over connection 108e to ONUs 1044 and 1045. The ONUs 1044 and 1045 converts the electrical signal to an optical signal for transmission to ODN 110 over connection 108o. ODN 110 combines the data cells D and E into a single cell stream 302. To prevent data collisions, APON protocols require ONUs 104 to transmit data cells at specific times and in short bursts. Thus, the laser diode (not specifically shown) associated with ONU 104 typically transmits a cell lasting a fraction of a microsecond or a burst of cells lasting a few microseconds, but may only transmit infrequently. Also, the laser diode typically transmits regularly, but may be powered down for an indeterminate length of time, which may change the laser diode""s operating characteristics including the laser diode""s operating temperature. Because the subscriber side laser diode may only transmit data infrequently and when it does transmit data the transmission is only a short burst of information, it is difficult to control the laser power input and light intensity.
As can be seen from the above, it would be beneficial to provide improved methods and apparatuses for measuring and controlling the power and intensity of the laser diode associated with the passive optical networks.
The foregoing and other features, utilities and advantages of the invention will be apparent from the following more particular description of a preferred embodiment of the invention as illustrated in the accompanying drawings.
To attain the advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, methods to drive a laser in a passive optical network are provided. In particular, the laser is driven at a current sufficient to drive the laser to a predetermined light intensity. The current for the predetermined light intensity is measured and the current necessary to drive the laser at zero intensity is extrapolated.
The present invention further provides methods for pre-driving the laser prior to a transmission grant. The pre-drive method comprises storing a logic 0 drive current and supplying a pre-drive current to the laser.